This invention relates to a scanning optical device used in an image forming apparatus such as a laser printer or the like.
Generally, a scanning optical device includes a light source which emits a beam and a rotatable polygon mirror which reflects the beam so that the beam scans across a surface of a photo-conductive drum. An f.theta.-lens is provided between the polygon mirror and the photo-conductive drum, which converges the beam on the surface of the photo-conductive drum. Hereinafter, the direction in which the beam moves according to the rotation of the polygon mirror is referred to as a main scanning direction. The direction perpendicular to the main scanning direction on a mirror surface of the polygon mirror is referred to as an auxiliary scanning direction.
A `scanning range` is set within the actual moving range of the beam caused by the rotation of the polygon mirror. When the beam reaches one end (a starting position) of the scanning range, the scanning optical device starts modulation of the beam, so as to form latent image on the surface of the photo-conductive drum. The modulation of the beam is continued until the beam reaches the other end (an end position) of the scanning range.
In order to detect the beam reaching a proximate position to the starting position of the scanning range, an SOS (start-of-scan) sensor is provided in the scanning optical device. The SOS sensor is so constructed as to receive the beam via an intermediate mirror disposed in the proximity of the f.theta.-lens.
Since there is a possibility that the rotation axis of the polygon mirror is inclined due to a manufacturing error, the SOS sensor must have a length in the auxiliary n scanning direction. Thus, the SOS sensor uses a PIN photo diode array (as a beam detector)that has a plurality of elongated light receiving surfaces arranged in the main scanning direction. Each light receiving surface of the PIN photo diode array extends in the auxiliary scanning direction.
Further, in order to compensate the deviation of the rotation of the photo-conductive drum, a recently developed optical scanning device has a dynamic prism that is moved so that the beam shifts in the auxiliary scanning direction. It is preferred to dispose the dynamic prism between the light source and the f.theta.-lens, because the amount of the necessary movement of the dynamic lens can be decreased as the dynamic prism is close to the light source. In such a case, when the dynamic prism is moved, the beam directing toward the SOS sensor is also shifted of the beam in the auxiliary direction.
In such a scanning optical device, if the light receiving surface of the SOS sensor is inclined with respect to the main scanning direction, the following problem may arise. FIG. 1 shows the light receiving surface PD and a scanning line, that is, the movement of the beam passing through the light receiving surface PD. The scanning line before the dynamic prism is moved (that is, when the dynamic prism is positioned at its original position) is indicated by an arrow `a`. The timing when the beam moves across the light receiving surface PD is indicated by Ta. When the dynamic prism is moved, the scanning line shifts in the auxiliary scanning direction as indicated by arrows `b` and `c`. If the scanning line shifts as indicated by the arrow `b`, the timing when the beam move across the light receiving surface PD is changes to time Tb that is ahead of time Ta. Conversely, if the scanning line shifts as indicated by the arrow `c`, the timing when the beam move across the light receiving surface PD is changes to time Tc that behind time Ta. Accordingly, the timing of the beam detection by the SOS sensor is influenced by the movement of the dynamic prism. Conseqently, the detected timing when the light modulating is to be started is influenced by the movement of the dynamic prism.